This invention relates generally to carbon nanotubes and nanofibers, and more particularly to methods for fabricating multiple-walled carbon nanotube and carbon nanofiber film materials.
Carbon nanotubes are cylindrical carbon molecules and exhibit numerous useful properties, including exceptional mechanical strength, unique electrical properties, and efficient thermal conductors. Such properties are desirable in many applications, including nano-electronics, optics, and materials of construction applications. More specifically, the exceptional qualities of carbon nanotubes are desirable in the development of multifunctional and smart composites and for use in variety of instruments, sensors, and other devices.
There are primarily two categories of nanotubes: single-walled nanotube (SWNT) and multiple-walled carbon nanotube (MWNT) materials. SWNTs typically have small diameters (˜1-5 nm) and large aspect ratios, while MWNTs typically have large diameters (˜5-200 nm) and small aspect ratios. Another type of nanoscale material is a carbon nanofiber (CNF) material. CNFs are filamentous fibers resembling whiskers of multiple graphite sheets.
One significant advantage that MWNTs and CNFs have over SWNTs is that the fabrication of MWNT and CNF materials costs substantially less than the fabrication of SWNTs. For example, the fabrication of MWNT and CNF may cost approximately $10-20 per gram and $50-120 per pound, respectively, while the fabrication of SWNT may cost approximately $250-1,000 per gram.
However, despite their cost advantage, MWNTs and CNFs alone are difficult to form into sizeable and flexible film materials using current techniques due to their relatively large diameter, small aspect ratios, and stiffness. NanoLab (Newton, Mass.) has demonstrated the feasibility of making small-diameter MWNT (˜1 inch to 5 inch) material films through suspension and filtration of MWNT suspensions. Shinshu University, Japan, also demonstrated the possibility of making buckypapers from pure small diameter, double-walled nanotubes (DWNTs) (see Endo et al., Nature, 433:476 (February 2005)). However, the manufacturability of making large and flexible MWNT films has not been demonstrated. Other techniques for making MWNT and CNF material films generally require chemical bonders or agents. The use of chemical bonding agents, however, is undesirable due to the difficulty of eliminating the residual chemicals from MWNT and CNF films. The residual chemical bonding agents may also detrimentally affect the material properties of the resulting films. For instance, the residual agents may reduce the permeability and electrical conductivity of the CNF film.
It therefore would be desirable to provide improved methods for fabricating MWNT and CNF material films, including such methods that do not require use of chemical bonding agents. It also would be desirable to provide more useful MWNT and CNF material films, such as ones that are relatively flexible.